When choosing an antihistamine—or trying to figure out where a new symptom came from—be aware that the medication can come with side effects, including some that may be a little surprising. Here are some of the most common antihistamine side effects that you should know about.
Feeling drowsy or sleepy is one of the most common antihistamine side effects, especially with regards to first-generation antihistamines. It’s so common, in fact, that diphenhydramine is actually used as a sleep aid in some medications. This side effect obviously isn’t great if you have to drive or operate heavy machinery after taking your allergy meds, but Dr. Lane says that it can be beneficial in other circumstances, like if your allergy symptoms are making it difficult for you to get to sleep at night. Just be aware that it can cause some grogginess in the morning as well, he says.
However, if that’s starting to become a regular issue, you may want to talk to your doctor about other options. Having severe sleep or allergy issues every single night probably warrants a better insomnia- or allergy-management strategy.
Newer antihistamines tend not to cause as much sedation, but they still can in some people, Dr. Lane says. In his experience, Zyrtec has the highest incidence of sedation followed by Xyzal, Claritin, and Allegra, respectively.
2. Dry mouth
Dryness, especially dry mouth, is another very common side effect of antihistamines, Dr. Lane says. Again, this one is most common with the older medications, but can happen with the newer ones as well. The current thinking is that it’s the effects on acetylcholine that drive this side effect, which explains why it’s more common with the first-generation drugs.
This particular side effect generally isn’t serious, but if you’re trying to manage dry mouth for other reasons, know that your antihistamine could be contributing to it.
3. Dry eyes
Similar to dry mouth, dry eyes is also a common antihistamine side effect. It’s thought that some antihistamines can actually reduce your tear production via effects related to acetylcholine, causing your eyes to feel more dry. That’s especially annoying because dry eyes can also be red, stinging, and irritated, which are also common symptoms of itchy eyes due to allergies. Drying out allergy-affected eyes further can sometimes just make the symptoms worse.
Yep, antihistamines can have an effect on your bowel habits as well! Again, this is down to the medications’ effects on acetylcholine signaling. Normally, this neurotransmitter plays a role in the control of your gut muscles, which help move things along. If that movement is slowed thanks to a medication messing with acetylcholine, you might get a little constipated. But, as with the other acetylcholine-related side effects, this one is more common with first-generation than second-generation antihistamines.
5. Urinary retention
Increased urinary retention—meaning that your bladder doesn’t empty all the way when you pee—is another possible side effect of antihistamines that’s related to their actions on acetylcholine. Similar to the way the drugs can affect the gut muscles, they can also affect the bladder muscles, making it difficult to fully empty the bladder.
6. Dry skin
While not as common as dry mouth or dry eyes, dryness affecting your skin can be a side effect of antihistamines, Dr. Lane says. Generally, though, dry skin is affected more directly by other factors, such as changes in the weather or your environment.
7. Tolerance (sort of)
If you feel like the antihistamines you’ve been taking aren’t working as well as they used to, you’re not alone. Experts aren’t sure exactly what’s at the root of this issue (whether we’re really building up a true tolerance to the medication or our symptoms are just getting more severe, for instance), but it’s one that allergists see frequently, Dr. Lane says.
Luckily, if you’re using OTC medications, this problem has a pretty easy fix: switch to a different one! In fact, Dr. Lane says that some people are able to switch to a different medication for a while and then switch back to their old standby a few months or years later if they start to have problems with the newer one. So, this issue isn’t always permanent, he says.
8. Reduced lubrication
It’s not one of the more common antihistamine side effects, but reduced vaginal lubrication is a possibility, SELF explained previously. Vaginal dryness is a known side effect of anticholinergic drugs, including antihistamines with anticholinergic effects, because these drugs can cause changes in blood flow that dry out the mucus membranes in the body.